| Steroid hormones mediate avian behavior by acting on target tissues throughout the body, including the central nervous system. The steroidal state of birds typically is measured as blood plasma concentration, but plasma values may not accurately represent steroid concentrations within tissues. I measured steroid concentrations within the testes and brains of male cinnamon teal (Spatula cyanoptera), mallards (Anas platyrhynchos), chukar (Alectoris chukar), Eurasian collared-doves (Steptopelia decaocto), and European starlings (Sturnus vulgaris) collected during the breeding seasons of 2019-2021. I found similar testosterone concentrations between the left and right testes in all species. Testicular testosterone maintains testicular function, but also may be exported to systemic circulation. I found similar testosterone and estradiol concentrations in four brain regions in all species except chukar. Steroids mediate brain function through local regulatory mechanisms and these data suggest tight control of steroid concentrations in neural tissues. |